Surface Treatment – Necessity Or Luxury?

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Vorbehandlung Pre-treatment

Claudia Benelli, Director, Ferrarini & Benelli Surface Treatment – Necessity or Luxury?

enerally, plastic films have inert and non-porous chemical tribute a given level of applied power. Each type of electrode has Gsurfaces, with low surface tensions, which make them non- a maximum power limit it can accept for length units. If a given receptive to the anchoring of substrates such as printing inks, power density KW exceeds the maximum limit of what an elec- adhesives and coating. Amongst different plastic films, polyeth- trode can distribute, it will be necessary to add other electrodes. ylene and polypropylene films have the lowest surface energy Increasing the number of electrodes could impose adoption of a and therefore undergo corona treatment more often, in order to discharge roller with larger diameter. Both factors lead to an enhance their adhesion properties. However, corona-effect sur- increase in the size of the treatment station to be used and its face treatment is not limited to these two materials and can also cost. be used to enhance adhesion properties of basically all plastic Operative speed is another system parameter, which can com- materials as well as some materials of other nature. Two non- plicate calculations for plant sizing. With a given plant, the plastic materials that undergo surface treatment more frequent- higher the operative speed, the lower the maximum power den- ly are aluminium and paper. sity attainable. Thus, speed, which is inversely proportional to All substrates, plastics films, paper and aluminium provide best power density, has a relevant impact on sizing and cost of a surface adhesion when treated at the time of their manufactur- corona treatment system. ing. Such application, which is called post-treatment, is used to intensify adhesion during other converting processes such as PROCESS-MATERIAL PARAMETERS. Most apparent process-mate- printing, lamination, etc. rial parameters are substrates composition and type of process they undergo (extrusion, extrusion coating, printing, etc.). FACTORS AND PARAMETERS. The ultimate goal of every surface Many materials are defined by a range of typical surface ten- treatment system is to increase product surface tension, meas- sions. Different surface tension values of a given material could ured in dyne/cm as well as to enhance wettability and surface be explained by several factors, such as processing method adhesion properties. Corona treatment systems achieve such (blown or cast film), die temperature variations during extru- results by applying a given power, for a given time on the sur- sion, type and quantity of impurities that are always present face. This power parameter in time is measured in watt densi- even in high-quality films. If dealing with a material having a ty, defined as Watt*minute/m2. given power density its surface tension will be increased. Yet final surface tension and increase amount depend on the mate- Wd=PS(w)/WW(m)*LS(m/min)*NST rial’s initial surface tension. For example, by applying a given watt power density to a 41 Wd= watt power density (watt/m2/min) dyne activated PET, it will be possible to reach 46 dyne, but PS(w)= supplied power (watt) applying that same power density to a 44 dyne activated PET, WW(m)= film width (m) only 48 could be attained. Although in the latter case the final LS(m/min)= line speed (m/min) level is higher, the increase is inferior due to the initial level. NST= number of sides to be treated Furthermore, the variation of barefoot material response to corona treatment is compounded by additive loading. CONDITIONAL PARAMETERS OF WATT DENSITY. There are two Not all materials or substrates react in the same way to corona parameters that affect power density: system and process-mate- treatment. Some materials, such as certain polyesters, can be rial parameters. treated without problems and show rapid surface tension increase using relatively low power intensity levels. For other SYSTEM PARAMETERS. Most obvious system parameters are materials, such as polyethylene, treatment poses more difficul- measurement of supplied power (KW) and measurement of ties, however, significant increases in surface tension can be treatment station (film width). Applied power density is direct- reached using medium-low levels of power intensity. Finally, ly proportional to measure of supplied power in watt and some materials, such as polypropylene, are difficult to treat and inversely proportional to treatment station measure expressed may show only moderate surface tension increase using rela- in meters; this means that in order to maintain a given power tively high power intensity levels. density, if the film width doubles even the watt quantity of sup- Reaction of a material to corona treatment is also affected by the plied power must be doubled. This direct relation is complicat- different manufacturing process parameters. It is known that ed by two factors: operative speed and electrode capacity to dis- extrusion, extrusion coating, and lamination by extrusion

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Vorbehandlung Pre-treatment

processes require different treatment levels to reach acceptable product and production quality levels. Nevertheless, it is less known that the type of manufacturing affects reactions of a given substrate to corona treatment. These differences are due to the following elements:

1. Variation in molecular structure as result of the extrusion process. 2. Substrate temperature at the time it enters the corona treatment unit. 3. Location of the corona treatment unit relative to the point of extrusion. UV/EB – Green Technology ADDITIVE CHARGE. Additive charge, expressed in parts per mil- lion (ppm) contained in the film, remarkably influences a film’s for Innovation capacity for corona treatment and retains its effects in time. The first consequence of an elevated additive charge is that of requiring an increase in power density to enhance the film surface tension. Secondly, an elevated quantity of additive in the film reduces a material’s capacity to maintain a corona treatment effect in time. This is caused by the additives’ tendency to group together or migrate to the surface and mask the effect of corona treatment. It is commonly believed that additives will migrate more rapidly towards a film surface, which has been Conference & Exhibition treated. October 18 – 20, 2011 in Basel | Switzerland

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The diagram in Figure 1 shows that the possibility of treatment decreases with the increase of the lubricant percentage. The example shows that about 16 W*min/m2 are necessary to treat a PE film with 300 ppm of lubricant at 42 dyne/cm. If the lubricant content were 600 ppm the amount of necessary corona effect would rise to 25 and if it were 1200 ppm it would The ofﬁcial event of increase to 40 W*min/m2. For a material not containing lubricants, an amount of corona effect equivalent to about 7W.min/m2 is applied. This means that for treatment of films containing lubricants, European Association for increased electric power must be applied compared to that nec- the promotion of UV&EB essary for treatment of lubricant-free materials. For the same curing technology result, electrodes with increased discharge surface are necessary, since an increased discharge electrode surface with equivalent film sliding speed causes increased film exposure time Vincentz Network · Nicole Steinbach under corona discharge. Plathnerstr. 4c · 30175 Hannover · Germany Tel. +49 511 99 10-274 · Fax +49 51199 10-279 [email protected] TREATMENT RENEWAL. Treatment renewal is that situation, in which a film that had previously undergone treatment is treat-

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Vorbehandlung Pre-treatment

Fig.2: Polimetal Corona Treatment

ed again in-line with the corona process before completion of Fig.3: Corona Generators the converting process. For instance, a film already treated for extrusion could be treated again in the laminating, coating or process. Yet an elevated initial treatment level is not a solution printing process. to the problem. Treatment renewal is more commonly used in the presence of It must also be said that, if a material contains added compo- water-based coatings or inks; on the contrary, solvent-based nents such as additives and slipping agents, treatment at the coatings or inks wet more easily and their adhesion is simpler. time of use is more efficient, if the material has already been For example, a film with 31 dyne surface tension at extrusion treated at the time of extrusion. This case is called treatment can be treated to a level of 40 dyne in line with the extruder. reinstatement or renewal. After transport and stocking, the effect of time passing and additive charges, surface tension is reduced to, for example, Ferrarini & Benelli Srl., 36 dyne or even less. At this level, solvent-based coatings or I-26014 Romanengo (CR), inks would already provide elevated product quality, without www.ferben.com needing further treatment. However, a water-based coating or ink would require the film to be treated to increase its surface tension to 40 dyne or more. Experience proves that, with a given surface tension increase, re-treating pre-treated films requires less power intensity than treating a non pre-treated film. Moreover some non pre-treated materials do not increase their surface tension even if very elevated power densities are applied. This is due to their molecular structure and slipping barrier or additives, which migrate, to the film surface, reduc- ing surface tension.

TIME. With time treated surfaces progressively lose the effect of the treatment. This is always true for all treatment methods and all films, even non-additivated ones. The amount of treatment loss depends on the type of film, treatment level, type of treatment, additive quantity and elapsed time. Unfortunately some of the beneficial results obtained by the corona treatment degrade after treatment. Main reasons for this are:

1. It is thought that the main cause for this loss is that each subsequent rubbing of the treated surface on transmission rollers, reel, drum, etc., tends to «remove» treatment. 2. Contact between treated side and opposite side. 3. Content of slipping agents.

It should also be noted that once final processing, such as printing, coating or lamination is over, anchor enhancement obtained with corona treatment remains stable. In other words, ink will not come off after a period of time and the two substrates will not delaminate. Treatment «loss» occurs in the period of time between treatment and the consequent converting

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